1. Field of the Invention
The present invention relates to receivers for digital broadcasting which uses multi-carrier transmission methods such as OFDM (Orthogonal Frequency Division Multiplexing), FDM (Frequency Division Multiplexing), etc., and particularly to a synchronization control device for data frame structure and individual transmitted symbols.
2. Description of the Background Art
The OFDM (Orthogonal Frequency Division Multiplex) transmission system is known as a method for enabling transmission of digital data to mobile objects largely affected by problems in radio propagation such as multipath and fading, whose utilization for broadcasting is being developed. Typically, it includes the digital Audio Broadcasting (hereinafter referred to as DAB) provided by ITU-R recommendation BS.774.
FIG. 14 is a block diagram of a conventional digital broadcasting receiver. In this diagram, 1 denotes an antenna, 2 denotes an RF amplifier, 3 denotes a frequency converter, 4 denotes a local oscillator, 5 denotes an intermediate frequency filter, 6 denotes an intermediate frequency amplifier, 7 denotes a quadrature demodulator, 8 denotes an intermediate frequency oscillator, 9 denotes an A/D converter, 10 denotes a alignment signal detector, 11 denotes a data demodulator, 12 denotes a control device, 13 denotes an error correcting code decoder, 14 denotes an MPEG (Moving Picture Experts Group) audio decoder, 15 denotes a D/A converter, 16 denotes an audio amplifier, and 17 denotes a speaker.
In the receiver constructed as explained above, broadcasting wave received at the antenna 1 is subjected to amplification in the RF amplifier 2, frequency conversion in the frequency converter 3, removal of unnecessary components such as adjacent channel waves in the intermediate frequency filter 5, amplification in the intermediate frequency amplifier 6, detection in the quadrature demodulator 7 and then applied to the AID converter 9 as baseband signal.
The signal sampled by the A/D converter 9 is demodulated in the data demodulator 11. Specifically, the processings performed there include phase detection of each transmission carrier subjected to Quadrature Phase Shift Keying (QPSK) by complex discrete Fourier transform processing (hereinafter referred to as DFT processing) for each transmission symbol and differential demodulation based on the co-carrier modulation comparison between two transmission symbols adjacent in time. The OFDM demodulated data is sequentially outputted to the error correcting code decoder 13 according to the carrier order rule for modulation on the transmitting end.
The error correcting code decoder 13 releases time interleaving over a plurality of transmission symbols made on the transmitting end and decodes the data transmitted in the form of convolutional codes. At this time, errors in the data, which took place in the transmission path, are corrected.
In the decoded data in the error correcting code decoder 13, audio data is outputted to the MPEG audio decoder 14 and control data indicating contents and structure of the transmission data is outputted to the control device 12. The MPEG audio decoder 14 expands DAB audio data compressed according to the rule of ISO/MPEG1 layer 2 and provides the data to the D/A converter 15. The audio signal, analogue-converted in the D/A converter 15, is reproduced from the speaker 17 through the audio amplifier 16.
The alignment signal detector 10 detects a null symbol (=a period without signal) in the frame alignment signal in the DAB transmission signal shown in FIG. 16 by envelope detection, whose output is applied to the control device 12. The control device 12 estimates timing of following transmission symbols on the basis of the null symbol timing to provide control so that the data demodulator 11 can correctly apply DFT processing to each symbol.
While rough synchronization is thus established with the demodulation signal, the synchronization processing is based on the envelope detection of the null symbol provided at the head of a frame and hence it is difficult to obtain correct timing in the case where reflected wave or noise overlaps the signal. Accordingly, more correct synchronization processing is performed on the basis of the phase reference symbol which is provided as part of the alignment signal and in which modulation of each carrier is known.
FIG. 15 is a block diagram of a timing synchronization processing device in a conventional digital broadcasting receiver. The timing synchronization processing device is included in the control device 12 of FIG. 14. In this diagram, 101 denotes phase correcting means, 102 denotes phase reference symbol data holding means, 103 denotes IDFT (inverse discrete Fourier transform) means and 104 denotes peak detecting means.
In this synchronization processing, the input of the phase correcting means 101, provided from the OFDM demodulator 11 in FIG. 14, is the result of DFT processing of demodulation signal of the phase reference symbol, which can be expressed as shown below (data corresponding to an mth carrier). The expression (1) represents demodulation data of the mth carrier provided as the result of the DFT processing. The DFT processing is a complex FFT, whose result is complex number data having a real part and an imaginary part. The expression (1) represents the complex number data in terms of amplitude and phase. Note that effect of noise is deleted here. ##EQU1##
Where, .theta.m represents modulation phase, t.sub.e represents a timing shift or timing error of the sampling starting timing in the data demodulator 11 from the effective symbol starting timing, and A.sub.m represents amplitude of demodulation data of the mth carrier. In the case of DAB, since carriers are modulated by QPSK, the amplitude A.sub.m is approximately constant irrespective of the numbers of the carriers.
The phase correcting means 101 multiplies the input data, by using known phase reference symbol data provided from the phase reference symbol data holding means 102, by its complex conjugate value. In the ideal condition, this releases phase modulation applied to each carrier of the phase reference symbol and all carriers thus match in phase in the output of the phase correcting means 101. The complex conjugate value of the phase reference symbol modulation data can be represented by the minus j.theta.mth power of the natural logarithm e. The term including .theta.m in the expression (1) is thus eliminated in the output of the phase correcting means 101.
Next, in the IDFT means 103, inverse discrete Fourier transform processing (hereinafter referred to as IDFT processing) is applied to the output of the phase correcting means 101. Then, in the output transformed into the time region, an impulse signal is generated at the point where all carrier phases coincide, i.e., at t=t.sub.e.
The peak detecting means 104 detects the impulse signal appearing in the output of the IDFT means 103 on the basis of amplitude of each data. The position of the impulse signal detected there approximately corresponds to the time gap of t.sub.e, which represents a time gap between the sampling starting timing in the data demodulator 11 and the starting timing of the phase reference symbol. Therefore, adjusting timing so that the position of the impulse signal is at the head of the IDFT processing result enables correct timing synchronization processing.
In the timing synchronization processing device explained above, a large amount of operations are required in the above-described processings when a large number of carriers are used for transmission (1,536 carriers are used in the DAB transmission mode 1 and 2,048 point processing is done in IDFT), which results in large circuit scale. Or, the necessity of high-speed operation increases power dissipation.